The present invention relates to an anode of a fusion electrolysis furnace for the production of aluminum, which anode consists of a plurality of individual oxide-ceramic elements of stable dimensions.
The currently used Hall-Heroult process for obtaining aluminum for alumina dissolved in cryolite takes place at 940.degree.-1000.degree. C., and electrolysis is carried out between a horizontal anode and a parallel liquid aluminum cathode. The anodically precipitated oxygen reacts with the anode carbon to form carbon dioxide and the carbon burns away. At the same rate as the linear burning away of the anode takes place, in the case of suitable cell geometry the build-up of the aluminum layer takes place cathodically, so that the interpolar distance is maintained. After the scooping of the liquid aluminum the interpolar distance must be freshly adjusted by lowering of the anodes. Furthermore, burned-away carbon anode blocks must be replaced at regular intervals. A special works, the anode factory, is necessary for the production of these anode blocks.
Replacement of the burning carbon anodes by an oxide-ceramic anode of stable dimensions should, in comparison with the conventional Hall-Heroult process, bring a whole series of advantages:
Simplification of furnace operation; PA1 Reduction and improved detection of the furnace waste gases; PA1 Independence of the fluctuations of price and quality of the petroleum coke; and PA1 Lower overall energy consumption of the process. PA1 The production of ceramic bodies of large format; PA1 The insertion and manner of operation in the electrolytic cell without mechanical damage to the ceramic bodies; and
These factors should take effect in reduced metal production costs.
Oxide-ceramic anodes of stable dimensions which are used in cryolite melts are known and are disclosed for example in Ger.Pub.Sp. 24 25 136. In further publications whole classes of substances for use as oxide-ceramic anodes are described, for example spinel structures in Ger.Pub.Sp. 24 46 314 and Jap. publication specification 52-140 411 (1977). In Jap. publication specification 52-153 816 (1977) finally an oxide mixture of the composition Zn.sub.1.7 Ni.sub.0.3 SnO.sub.4 is proposed which is applied to a wire mesh, whereby a gas-permeable porous electrode is formed.
The multiplicity of proposed metal oxide systems indicates that hitherto it has not been possible to find an ideal material which satisifies the many, in some cases contradictory, demands of cryolite electrolysis, and which is economical.
In the replacement of the currently utilized carbon blocks of large format of the Hall-Heroult electrolytic cell by ceramic anodes of stable dimensions and of good conductivity, three main difficulties arise:
The achievement of long life with minimum possible anode corrosion.
Replacement of the carbon anodes by ceramic anodes signifies that several tons of ceramic material must be mixed, ground, pressed and sintered. The resultant anode bodies should differ as little as possible in their physical properties. In Ger.Pub.Sp. 24 25 136 it was therefore proposed to embed individually produced anode blocks of oxide-ceramic material in an electrically insulating carrier plate resistant to the melt. The individual anode blocks are in contact with a current-distributor plate. The ceramic anodes can be inserted into the carrier plate in such a way that they are flush with the lower plane of the carrier plate or protrude from it. The removal of the generated anode gas is facilitated in that individual apertures in the carrier plate are not fitted with anode blocks (FIGS. 5 and 6).
The Figures also show that the anodes are designed so that both the carrier plate and the oxide-ceramic material are dipped into the melt.
In the insertion of the anodes into the melt and in the case of temperature fluctuations in operation, axial and radial temperature gradients occur which cause mechanical tensile stresses which can even lead to tearing of the carrier plate fitted with oxide-ceramic blocks.
The erosion of the ceramic metal oxide is effected substantially by the aluminum present in the cryolite. Thus, the anode corrosion is dependent upon the conveying of substance from the melt to the solid body, which is mainly a function of the escape of the anodically generated gas. The desired gas outflow is only partially achieved by the arrangement of regularly distributed holes in the carrier plate according to Ger. Pub. Sp. 24 25 136, especially with ceramic anodes protruding from the electrically insulating carrier plate.